As known by those skilled in the art, a conventional analog PLL may be susceptible to errors (or even error propagation) since said analog PLL uses analog operations and analog elements. Therefore, digital phase-locked loops (DPLL), which utilize a counter with a variable divisor on the feedback path, are proposed for relieving the errors with the partial aid of digital operations and digital elements, and moreover, an all-digital phase-locked loop (ADPLL) may significantly help in area reduction and process migration. For example, a digitally-controlled oscillator (DCO) may be used to replace the conventionally used voltage-controlled oscillator (VCO), which is an analog element. A phase detector may also be replaced with a time-to-digital converter. Therefore, ADPLL is gaining popularity and becoming a trend in radio communications.